The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted to be prior art against the present disclosure.
In magnetic recording, as one example, reading and writing are performed by one or more heads that move relative to the surface of a storage medium. Many magnetic disk drives, for example, include a plurality of individual disks, or “platters,” which may be two-sided—i.e., each platter can store data on each of its two sides. Therefore, such a disk drive would have at least two heads for each platter. Indeed, for each platter, there is normally at least one write head and at least one read head, so that such a disk drive normally has at least four heads per platter. Although the read heads and write heads are separate, they may share some structures, and they may be mounted on a common substrate.
In a common configuration, all of the heads in a given disk drive are mounted on arms attached to a common actuator that controls the radial position of the heads (a circumferential component of motion is provided by the rotation of the platters relative to the heads). That means that all of the heads in a disk drive generally move together. While some disk drives include “dual-stage” actuators that provide some degree of independence to the different heads on the common actuator, the degree of independence may be limited.
In order to control the radial position selected by the actuator, each surface of each platter has distributed upon it positional information referred to as “servo” data. The servo data are commonly distributed in spaced-apart servo “wedges” (generally spaced equiangularly) on the platter surface. By reading the servo data as each servo wedge passes under the read head, the disk drive controller can determine the precise radial position of the head and can feed back that determination to control the position of the read head or the write head, depending on the required operation.
In most read or write operations, if data are to be written to, or read from, different platters in a disk drive, it is unlikely that the data on the different platters will be at the same position on each platter (a collection of positions at a single radius on a single side of a single platter is referred to as a “track” of that side of that platter; a collection of tracks at nominally identical radial positions on all of the sides of all of the platters of a disk drive is referred to as a “cylinder” of the disk drive). Therefore, it is common for the actuator to move all of the heads to a particular cylinder to accommodate reading or writing by just one of the heads of just one track on one side of one platter.
If other operations are queued up to access other tracks, those other operations would wait. Even if there is another operation that will access a different track in the same cylinder (i.e., the same cylinder but a different platter), because there is only one actuator controlling all heads, and because the radial position of each head may vary slightly (e.g., because of environmental conditions such as vibration, or thermal gradients that distort the actuator and/or arms), normally that other operation still has to wait because the servo operation can only move the actuator to one position at a time (unless there are dual-stage actuators as noted above, and the difference in position from platter to platter is within the range of independent motion provided by the dual-stage actuators).